VOLUME 24
JANUARY 2002
Prominent Hill Cu–Au discovery
New diamond field discovered
New zinc discovery in the Flinders
Historic Native title agreement
Gemstones
New diamond field discovered
in SA
Steven Cooper (Senior Geologist, Orogenic Exploration Pty Ltd)
Introduction
6
MESA Journal 24
January 2002
750000
6400000
Sugarloaf Dam
#
Port Augusta
U%
U%
Streaky Bay
Kyancutta
U%
U%
U%
Whyalla
6300000
Kimba
Flinders
Island
Elliston
U%
#
Mount Hope
Wallaroo
U%
6200000
U%
6200000
Basement outcrop on the island is
Hiltaba Suite Calca Granite (~1456 Ma),
a holocrystalline coarse-grained,
red-brown, strongly feldspathic granite
which crops out on the wave-cut
platforms and rarely on the interior of the
island. Most of the island is veneered by
Pleistocene Bridgewater Formation
calcrete, which forms spectacular coastal
cliffs (Flint, 1992). There are some
Ho lo c ene coas tal sand dunes
(Semaphore Sand) and minor lacustrine
mud (Yamba Formation) in Gem Pan
(Fig. 2).
The island lies just south of the
east–west trending Polda Basin which
contains Neoproterozoic to Jurassic
sediments.
Sixty kilometres east of Flinders
Island, on the Eyre Peninsula mainland,
Stockdale Prospecting Ltd (now De
Beers Australia Exploration Ltd)
discovered a number of Jurassic
kimberlitic intrusions (Mt Hope 01 to
08) close to Elliston (Mitchell, 1992a,b).
Their locations were suggested by the
distribution of anomalous kimberlitic
650000
6300000
Regional geological
setting
550000
6400000
The Gawler Craton has, for some time,
been known to have potential for
kimberlite discoveries, Recently,
Orogenic Exploration Pty Ltd found a
complete range of kimberlitic indicator
minerals, and five small diamonds, on
Flinders Island, ~40 km offshore from
Elliston on western Eyre Peninsula (Fig.
1). While no source kimberlitic bodies
have been located to date, the presence of
fragile indicator minerals such as
diopside, olivine and phlogopite
suggests a local source, and confirms the
existence of a new diamond field in SA.
This paper discusses the exploration
techniques used, and compares the suite
of kimberlite indicator minerals from the
island with the assemblage of minerals
from the known barren kimberlite field
near Mt Hope on the adjacent mainland,
east of Elliston.
450000
Cummins
U%
Port Lincoln
201126_006
450000
550000
650000
750000
Fig. 1 Location of Flinders Island, and kimberlite occurrences on the mainland.
Monroe anomaly
Point Malcolm
Flinders
anomaly
Gem Pan
anomaly
Flinders
Paddock
Gem Pan
Gem Paddock
indicator mineral collected from surface
samples, along with magnetic anomalies
defined by detailed aerial magnetic
surveys. Their presence was confirmed
by drilling. No diamonds were recovered
by Stockdale, and this was later
confirmed from drilling carried out by
Diamond Ventures NL (Cooper, 1998).
Tenements
Dunstan’s Paddock
Seal
Point
Groper Bay
Dunstan’s
anomaly
31
Bryants
Bay
0
2
KILOMETRES
Pyrope
counts
0
187
Chromite
counts
0
Diamond site
Fences
201126-012
Fig. 2 Pyrope and chromite diamond
indicator counts on Flinders Island at end of
December 2001.
Flinders Island (~38 km2) is owned
freehold, with the exception of a thin
coastal strip (3.59 km2) managed by the
Department for Environment and
Heritage, and a 234 m2 area around the
lighthouse owned by the Federal
Government. The island is currently an
active sheep station with over 5000
sheep.
Orogenic Exploration Pty Ltd, a small
private diamond exploration company
bas ed i n Mel bour ne, ho l d s t wo
exploration licences over the island, one
Gemstones
covering the 800 m wide coastal reserve
(EL 2875; 24.19 km2), the other over the
remaining central area (EL 2577; 15.55
km2). Orogenic has in place an option
agreement with listed company Tawana
Resources NL, which in turn has an
agreement with De Beers Australia
Exploration Ltd.
Exploration
methodology
Traditional heavy mineral kimberlitic
indicator mineral sampling has been the
main exploration tool used on the island,
with 607 surface samples collected to the
end of December 2001. Early samples
comprised two bags (~30 kg) of surface
material screened in the field to -3.2 mm.
Most later samples, including the five
sa mples recover ing diam onds,
comprised one bag (average 15 kg) of
-1.6 mm material. As there is no drainage
developed on the island, surface
sampling has been restricted to soil loam
scrapes collecting deflation (wind
concentrated) material.
A detailed aerial geophysical survey
was flown over the island in June 1999
for Orogenic Exploration Pty Ltd as part
of the TEISA area A4 survey. Detailed
magnetic, radiometric and digital
elevation data were acquired at 50 m line
spacing. Extensive areas of the island
were recently covered by a 50 m spaced
detailed gravity survey to aid in defining
drilling targets.
Sampling results
Sampling on Flinders Island has shown
that the dispersion of indicator grains is
very restricted, and that more traditional
wide-spaced sampling programs would
probably overlook similar indicator
mineral anomalies. It confirms the need
for systematic sampling using closer
spacing within this environment.
A wide range of indicator minerals
has been recovered from the island,
including the traditional chromite,
pyrope, diopside, and picroilmenite
m ineral s , as w el l as f or s t er i t e,
orthopyroxene, phlogopite and diamond.
Four distinct indicator anomalies have
been defined to date — Gem Pan,
Flinders, Monroe and Dunstan’s. The
highest grain counts are centred around
Gem Pan, on the northwestern side of the
island, covering most of the western half
of Gem Paddock (Fig. 2).
Covering the northwestern corner of
Flinders Paddock, centred along the
northern coast, is the Flinders anomaly.
Like the Gem Paddock area, chromite
dominates the grain counts. There are
low but significant grain counts in the
area between the Gem Pan and Flinders
anomaly areas; these two areas may
therefore be related.
Further east, near Point Malcolm, is
the Monroe anomaly from which three of
the five diamonds were recovered. This
grain anomaly, which contains more
equal counts of chromite to pyrope, is
very discrete.
Located halfway down the island,
near the west coast, is Dunstan’s
anomaly. Counts here are low, but all the
indicator minerals (excluding diamond)
have been recovered.
Further isolated recoveries of various
indicator grains have been made across
the island; some of these will require
further sampling to define their
significance.
The chemistry of indicator grains
recovered from the island is indicative of
the presence of diamonds, which has
been confirmed by the recovery of five
diamonds from surface samples.
Representative microprobe analyses are
provided in Table 1 for two samples
collected near Gem Pan.
Chromite
Chromite is the dominant kimberlitic
mineral on Flinders Island; over 2500
grains have been recovered, and the
chemical properties confirmed by over
2300 microprobe analyses. A significant
number of grains fall into the diamond
inclusion field based on MgO, TiO2 and
Cr2O3 contents.
N i net y- f our per cent o f t h e
microprobed chromite grains have
Fe +++ /Fe ++ ratios which indicate a
kimberlite oxidation state favourable for
diamond preservation. This is in line
with the picroilmenite data.
Chromite grains from sample FX-241.
Grains are 0.3 mm across. (Photo 48401)
Table 1. Representative analyses of some grains recovered in two samples near Gem Pan — chromite (CHR), clinopyroxene (CPX), garnet (GNT),
ilmenite (ILM) and olivine (OL).
SAMPLE (FIS)
Mineral
12
CHR
12
CHR
12
CHR
12
CPX
12
GNT
14
CHR
14
CHR
14
CHR
14
CPX
14
GNT
14
ILM
14
OL
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Cr2O3
V2O5
NiO
Fe2O3
Na2O
0.17
0.15
17.34
13.38
0.06
14.21
0.00
52.31
0.13
0.11
2.67
0.00
0.29
0.76
7.90
16.48
0.53
9.29
0.20
62.15
0.24
0.19
0.00
0.04
0.10
1.72
7.78
20.11
0.16
9.17
0.02
60.09
0.21
0.08
0.00
0.03
51.73
0.25
2.75
4.05
0.15
14.87
22.98
0.24
0.01
0.06
2.36
0.39
40.53
0.10
15.40
3.91
0.34
21.33
4.86
11.13
0.00
0.00
2.94
0.02
0.14
0.11
16.82
13.58
0.10
13.68
0.01
52.52
0.11
0.16
2.23
0.00
0.18
0.22
5.84
16.99
0.11
10.69
0.00
60.62
0.19
0.12
5.67
0.01
0.12
0.08
6.94
14.82
0.14
10.16
0.00
67.45
0.18
0.11
0.00
0.02
51.83
0.22
2.74
1.70
0.12
16.75
22.98
0.51
0.05
0.06
2.87
0.21
39.88
0.08
13.58
3.60
0.37
18.56
8.28
13.41
0.08
0.03
3.33
0.02
0.02
49.21
0.64
19.30
0.15
13.74
0.00
11.04
0.42
0.33
5.92
0.02
40.50
0.01
0.02
9.16
0.12
50.15
0.07
0.01
0.01
0.39
0.00
Total
100.53
98.07
99.47
99.84
100.56
99.46
100.64
100.02
100.04
101.22
100.79
100.44
MESA Journal 24
January 2002
7
Gemstones
Pyrope
Pyrope is the next dominant indicator
mineral, with over 260 grains recovered.
These include mostly G9 lherzolitic
(mainly olivine–pyroxene rock) pyrope,
seven classical G10 harzburgitic (mainly
olivine–orthopyroxene rock) grains, and
minor low-chromium lherzolitic garnet.
This would indicate a lower proportion
of depleted harzburgite–dunite in the
mantle xenolith suite sampled by the
unknown kimberlite compared to
lherzolite.
The range of Cr2O3 in the peridotitic
g a r n e t is encouraging, with a
well-defined population between 6 and
15 wt% Cr2O3. The seven G10 pyrope
grains have significant Cr2O3 (all over 8
wt%) which plot within the more
restrictive G10 diamond field.
The only known distribution of other
G10 pyrope grains in SA is from the
northern Springfield Basin (five grains
around 6% Cr2O3), and one grain from
the Sugarloaf Dam kimberlite sill, 20 km
west-northwest of Port Augusta.
Picroilmenite
Over 120 picroilmenite (ilmenite with
MgO >4%) grains have been microprobed from Flinders Island. The Cr2O3
wt% contents of kimberlitic picroilmenite generally range up to 7%.
Significantly, 27 picroilmenite grains
recovered from the island plot with >7%
Cr 2 O 3 . The highest is a grain from
sample FIS 24 near Gem Pan, with
22.8% Cr2O3. Similar individual high
chromium picroilmenites are known
from inclusions in the diamonds from
Mwadui Kimberlite, Tanzania, with a
reported 16.2% Cr2O3 (Stachel et al.,
1998), and from the Venetia Kimberlite,
South Africa, with 12.03% Cr 2 O 3
(Viljoen et al., 1999). Viljoen et al.
believed that the very high chromium
picroilmenites are from metasomatised
peridotite xenoliths.
mineral that does not survive and travel
well in the Australian environment.
Olivine
Olivine is rarely used as a kimberlitic
indicator mineral as it occurs in
abundance in basalt and other ultramafic
rocks, but kimberlitic olivine does have
certain chemical characteristics that can
eliminate many other olivine-bearing
source rocks. Olivine also does not
survive long near the surface and its
presence usually indicates proximity to
source.
Over 60 olivine grains from Flinders
Island have been microprobed, and most
are close to the forsteritic end member
and contain significant traces of NiO
(average 0.32 w t %) . W hi l e not
completely diagnostic of kimberlite, the
results are very encouraging as the only
known regional bedrock lithology is
granite.
Diamond
Three small diamonds were recovered
from the June 2000 regional 400 m grid
sampling program from three individual,
widely spaced samples. Follow-up
sampling recovered a microdiamond in
each of two separate samples located
near one of the original diamond sample
sites within the Monroe anomaly area. It
is worth noting that a different laboratory
was used for the follow-up sample
and 48406)
processing, thus eliminating the
possibility that the first three diamonds
were sourced from contamination.
Diopside grain (0.3 mm across) from sample
FX-212. (Photo 48402)
Preservation index for diamonds is
high to very high based on the Fe2O3,
FeO and MgO contents of the recovered
picroilmenite grains. Thus, the oxidation
condition in the kimberlite would not
have resulted in significant resorption of
any diamonds present.
Diopside
Over 150 fresh diopside grains have been
recovered from the island. Diopside is a
8
MESA Journal 24
January 2002
(Top to bottom) Diamonds from sample sites
FX-003 (centre of island), FX-070 (Monroe
anomaly), and FX-149 (south of Dunstan’s
Anomaly). These three stones were in the
0.3–0.5 mm size fraction. (Photos 48404, 48405
The diamonds are small, but only the
0.3 to 1.0 mm fraction from the loam
samples was examined for indicator
minerals. No examination of coarser
fractions is possible as the samples
contain very little material above 1 mm.
No samples have so far been collected
specifically for microdiamonds from
Flinders Island.
Comparison with
kimberlite from Mt Hope
Olivine grain (0.3 mm across) from sample
FX-210. Note the well preserved crystal
edges. (Photo 48403)
The chemistry of the indicator minerals
from Flinders Island is very different to
that of indicator minerals recovered from
the mainland kimberlite occurrences
near Mt Hope. Since both fields are
located in close proximity on the Gawler
Craton, it is important to observe the
differences between the two mineral
Gemstones
Acknowledgements
2.5
This paper is published with the
permission of Orogenic Exploration Pty
Ltd and Tawana Resource NL. The
project has been greatly assisted by the
inputs of Tawana geologist Wolf Marx,
and consulting mineralogist Lai Ho Seet.
D r Si m on Shee f r om De B e e r s
Laboratory Services kindly provided the
photomicrographs of the indicator
grains.
2.0
1.5
Cr2O3 wt%
assemblages. Direct contrasts can be
seen between the chromite, picroilmenite and clinopyroxene compositions
shown on Figs 3–5. Further differences
in c lude the lack of diam ondinclusion-field chromites (see Sobolev,
1971) recovered from the mainland
kimberlite occurrences. Chromite grains
1.0
0.5
(a)
80
0
0
201126-015
60
Cr2O3 wt%
(b)
0
4
8
12
MgO wt%
16
20
80
Cr2O3 wt%
40
4
0
201126-013
TiO2 wt%
16
20
Fig. 3 (a) Plot of wt% Cr2O3 versus MgO, and
(b) Cr2O3 versus TiO2, for chromite grains
from Flinders Island (red) and Mt Hope
(black). Mt Hope data is from Cooper (1998).
25
Cr2O3 wt%
20
15
10
5
0
8
12
plotted on Figure 3 show that the
Flinders Island chromite forms a very
different population based on higher Cr,
and spreads of Mg and Ti contents
compared to that from Mt Hope.
Picroilmenites from the Mt Hope area
differ from those on Flinders Island in
that the latter have higher than average
MgO contents and substantially higher
Cr2O3 contents. Clear differences are
also observed between the diopside
samples recovered from both areas,
based on Al2O3 and Cr2O3 content.
60
20
Al2O3 wt%
Fig. 5 Plot of wt% Cr2O3 versus Al2O3 for
clinopyroxene from Flinders Island (red) and
Mt Hope (black). Mt Hope data is from
Cooper (1998).
40
20
4
0
201126-014
4
8
12
MgO wt%
14
16
Fig. 4 Plot of wt% Cr2O3 versus MgO for
ilmenite from Flinders Island (red) and Mt
Hope (black). Mt Hope data is from Cooper
(1998).
No fresh olivine was recovered from
kimberlite in the Mt Hope area.
Conclusions
Diamond indicator mineral grains have
been recovered from sites with very poor
sample conditions on Flinders Island.
The range of minerals and the number of
grains recovered have been outstanding.
This, combined with favourable grain
chem istr y and t he di s cover y of
diamonds, make the project very
exciting. It is now a matter of locating the
origin of the indicator grains and
diamonds, which is presumed to be a
kimberlitic source and probably not too
far distant, given the presence of fragile
minerals such as diopside, olivine and
phlogopite.
This discovery now upgrades the
overall potential of the Gawler Craton
for diamonds. The stark differences
between the Mt Hope and Flinders Island
indicator grain chemistries illustrate that
rapid lateral changes in diamond
potential does occur.
For further information contact
Steven Cooper (mob. 0428 100 995).
References
Cooper, S.A., 1998. Exploration Licence
2151, Elliston, western Eyre Peninsula.
Third annual and final report for period
ending 23 December 1998. Unpublished
report by Orogenic Exploration Pty Ltd
for Diamond Ventures NL, Melbourne.
South Australia. Department of Primary
Industries and Resources. Open file
Envelope, 9206 (unpublished).
Flint, R.B., 1992. ELLISTON, South
Australia, sheet SI53-6. South Australia.
Geological Survey. 1:250 000 Series —
Explanatory Notes.
Mitchell, M.S., 1992a. Progress and final
reports for Exploration Licence 1694A
and B, Venus Bay, periods 9/1/91 to
15/7/92, for Stockdale Prospecting Ltd.
South Australia. Department of Primary
Industries and Resources. Open file
Envelope, 8422 (unpublished).
Mitchell, M.S., 1992b. Progress and final
reports for Exploration Licence 1672,
Elliston, periods 31/8/90 to 15/7/92, for
Stockdale Prospecting Ltd. South
Australia. Department of Primary
Industries and Resources. Open file
Envelope, 8527 (unpublished).
Sobolev, N.V., 1971. On the mineralogy and
diamond content of kimberlites. Russian
Geology and Geophysics, 3:70-79
Stachel, T., Harris, J.W. and Brey, G.P.,
1998. Rare and unusual mineral
inclusions in diamonds from Mwadui,
Tanzania. Contributions to Mineralogy
and Petrology, 132:34-47.
Viljoen, K.S., Phillips, D., Harris, J.W. and
Robinson, D.N., 1999. Mineral
inclusions in diamonds from the Venetia
Kimberlites, Northern Province, South
Africa. In: 7th International Kimberlite
Conference. Proceedings, 2:888-895.
MESA Journal 24
January 2002
9